1. Field of the Invention
The present invention generally relates to an electron microscope of scanning type. In particularly, the present invention concerns an improvement on detecting means for detecting secondary electrons emitted from a specimen in an electron-optical system of the scanning type electron microscope.
2. Description of the Prior Art
For having a better understanding of the invention, description will first be made on the scanning type electron microscope to which the invention can be applied as well as typical examples of the hitherto known secondary electron detecting means. FIG. 1 shows a general arrangement of the scanning type electron microscope in a vertical sectional view. Referring to the figure, electron flux emitted by an electron gun 1 is converged by means of convergence lenses 2 and 3 to an electron beam which is then deflected by scanning coils 4 and 5 so as to perform a two-dimensional scanning. The electron beam having passed through an objective lens 6 is focused upon a specimen 8 disposed on a specimen table 7 for the two-dimensional scanning thereof. Disposed in the vicinity of the specimen table or platform 7 is a secondary electron detector 9 which is adapted to detect secondary electrons emitted from the specimen 8 upon impingement of incident electrons. The detection signal output from the detector 9 is amplified and thereafter utilized for producing an image of the scanned specimen 14 on a screen of a cathode ray tube or Braun tube in synchronism with the scanning of the specimen with the electron beam. Reference numerals 10 and 11 denote apertures of the converging lenses 2 and 3, respectively, and numeral 12 designates an aperture for the objective lens 6.
In connection with the scanning type electron microscope of the structure outlined above, it is known that when a working distance w, i.e. a distance between a bottom plane 13 of the lower magnetic pole of the objective lens 6 and a top surface 14 of the specimen 8 is short, aberrations of the objective lens 6 are reduced, whereby the resolving power or resolution of the scanning type microscope is enhanced. However, when the working distance w is selected sufficiently short for the specimen 8 to be located within the magnetic field of the objective lens 6, at least a part of the secondary electrons emitted from the specimen 8 upon irradiation by the incident electron probe or beam will then be trapped in the magnetic field of the objective lens 6, as the result of which difficulty is encountered in detecting the secondary electrons with a reasonable efficiency by means of the secondary electron detector 9 which is positioned at a height lower than the bottom plane 13 of the lower pole of the objective lens 6, involving a serious problem.
As an attempt to solve the problem described above, it has been already proposed that an electrode 15 of funnel-like configuration is disposed in the vicinity of the specimen 8, wherein a voltage is directly applied to the specimen 8 with a positive potential being imparted to the electrode 15 so that a predetermined voltage difference makes appearance between the specimen 8 and the electrode 15, as is illustrated in FIG. 2. With such arrangement, the secondary electrons emitted from the specimen 8 is forcively caused to travel toward the secondary electron detector 9 under the action of the funnel-like electrode 15.
As another attempt to eliminate the afore-mentioned difficulty, there is known a proposal according to which the secondary electron detector 9 is positioned above the objective lens and the specimen 8 is intentionally disposed within the magnetic field of the objective lens 6 so that the secondary electrons S can be detected by the detector 9, as is shown in FIG. 3. With this arrangement, an image of the specimen can certainly be produced with an improved resolution.
However, the hitherto known arrangements of the secondary electron detecting means described above suffer many shortcomings. For example, in the case of the structure shown in FIG. 2, the range in which the specimen 8 is allowed to be tilted and/or displaced through a fine adjusting device (not shown) for manipulation of the specimen 8 is much restricted due to the necessary disposition of the funnel-like electrode 15 in the vicinity of the specimen 8. Further, there remains little freedom of selecting the positions at which detectors 16 and 17, which detect, for example, reflected electrons B and X-ray X as shown in FIG. 3, are to be installed, in addition to the detector 9. On the other hand, in the case of the arrangement shown in FIG. 3, a great deal of restriction is imposed on the size of the specimen itself as well as the range in which the specimen can be inclined and/or displaced, because the specimen 8 is disposed in the magnetic field of the objective lens 6.